Corresponding butterfly valves are used for example in volume flow rate controllers, which for example operate in a mechanically autonomous manner or comprise a control unit consisting of a transmitter, drive and controller. Here, the volume flow rate is normally determined via separate back-up elements. A static, dynamic or total pressure difference is tapped at these back-up elements and should have the best possible proportionality to the volume flow rate. Generally however, there is the problem that, with small volume flow rates, there is a high measurement inaccuracy in percentage. This is primarily due to the fact that there is a high spread of the differential pressure between Vmax and Vmin at the back-up elements, and the accuracy of the measurement devices is often determined by the maximum value.